Over the past two decades, new flat panel display technology based on light emission from thin layers of small organic molecules (organic light-emitting diodes or OLEDs) or conducting polymers (polymer light-emitting diodes or PLEDs) has emerged. Compared to liquid crystal-based displays (LCDs), this technology offers higher contrast displays with lower power consumption, and response times fast enough for video applications. Displays based on OLED and PLED technology exhibit a much wider viewing angle than liquid crystal displays (LCDs). Currently, more than seventy companies worldwide are developing display technologies based on OLED or PLED structures. Sales of displays based on OLEDs, such as car radios, mobile phones, digital cameras, camcorders, personal digital assistants, navigation systems, games, and subnotebook personal computers, are forecast to grow to more than one billion dollars in 2005.
The basic device configuration for both OLEDs and PLEDs is a multilayered or sandwich-type structure comprising a glass substrate, a transparent anode, two or more organic layers with different charge transport or luminescence characteristics, and a metal cathode. The morphology of the organic layers typically ranges from semi-crystalline to amorphous. Unlike inorganic LEDs, there are no lattice matching requirements with OLEDs and PLEDs, which greatly widens the types of substrates that can be used and the types of materials that can be combined together into devices. Use of multiple organic layers in the device geometry facilitates charge injection at the organic-electrode interface, leading to lower driving voltages. In addition, use of multiple organic layers allows the buildup of electrons and holes (and therefore, the location of the emission zone) to occur away from the electrodes, which significantly improves the efficiency of the device.
A typical organic light emitting device 30 in accordance with the prior art is shown in FIG. 1. The organic light emitting device 30 comprises a substrate 32, an anode 34 on the substrate 32, a hole transport region 36 composed of a hole transport material (HTM) on the anode 34, a mixed region 38 comprising a mixture of a hole transport material and an electron transport material on the hole transport region 36, an electron transport region 40 composed of an electron transport material (ETM) on the mixed region 38, a cathode 42 on the electron transport region 40, and a protective layer 44 on cathode 42. Not all electroluminescent devices have precisely the layers shown in the organic light emitting device 30 of FIG. 1. For convenience, the term “organic layer” used hereinafter to refer to multi-layer structures such as that of regions 36, 38, 40 as well as all structures of one or more organic layers that are equivalent to regions 36, 38, and 40. In the mixed region 38, one of the hole transport material and the electron transport material is an emitter. Upon application of an electrical current, the organic electroluminescent device radiates light generated by recombination of electrons and holes in the organic materials used to make regions 36, 38, and 40.
Hole transport materials used to form hole transport region 36 and mixed region 38 are typically a conductive material such as polyaniline, polypyrrole, poly(phenylene vinylene), porphyrin derivatives disclosed in U.S. Pat. No. 4,356,429, which is incorporated herein by reference in its entirety, copper phthalocyanine, copper tetramethyl phthalocyanine, zinc phthalocyanine, titanium oxide phthalocyanine, magnesium phthalocyanine, or mixtures thereof. Additional hole transporting materials include aromatic tertiary amines, polynuclear aromatic amines, as well as other compounds which are disclosed in U.S. Pat. No. 4,539,507 and U.S. Pat. No. 6,392,339, which are incorporated herein by reference in their entirety. The hole transport material may be deposited on the anode by, for example, plasma enhanced chemical vapor deposition (PECVD), atmospheric pressure chemical vapor deposition (CVD), sputtering, or spin-coating. See, for example, Van Zant, Microchip Fabrication, McGraw-Hill, 2000, which is incorporated herein by reference in its entirety.
Illustrative examples of electron transport materials that can be utilized in mixed region 38 and electron transport region 40 include, but are not limited to, the metal chelates of 8-hydroxyquinoline, as disclosed in U.S. Pat. Nos. 4,539,507; 5,151,629; 5,150,006 and 5,141,671, each incorporated herein by reference in its entirety. Another class of electron transport materials used in electron transport region 40 and mixed region 38 comprises stilbene derivatives, such as those disclosed in U.S. Pat. No. 5,516,577, incorporated herein by reference in it entirety. Another class of electron transport materials are metal thioxinoid compounds, illustrated in U.S. Pat. No. 5,846,666, incorporated herein by reference in its entirety. Yet another class of suitable electron transport materials for forming the electron transport region 40 and the mixed region 38 are the oxadiazole metal chelates disclosed in U.S. Pat. No. 6,392,339.
In some instances, mixed region 38 comprises from about 10 weight percent to about 90 weight percent of the hole transport material and from about 90 weight percent to about 10 weight percent of the electron transport material. The mixed region can be formed using mixtures of any of the suitable exemplary hole transport materials and electron transport materials described above, or other suitable materials known in the art. The mixed region 38 can be formed by any suitable method that enables the formation of selected mixtures of hole transport materials and electron transport materials. For example, the mixed region can be formed by co-evaporating a hole transport material and an electron transport material to form a mixed region. The thickness of the mixed region 38 affects the operational voltage and the luminance of the organic light emitting device. Mixed region 38 can comprise more than one layer. For example, mixed region 38 can selectively be formed to include two, three or even more separate layers.
Cathode 42 comprises any suitable metal, including high work function components, having a work function, for example, from about 4.0 eV to about 6.0 eV, or low work function components, such as metals with, for example, a work function of from about 2.5 eV to about 4.0 eV.
While FIG. 1 shows the basic structure of an OLED, variations of the structure shown in FIG. 1 are known. Such variations are disclosed in patents such as U.S. Pat. No. 4,356,429; U.S. Pat. No. 5,593,788; U.S. Pat. No. 5,408,109; U.S. Pat. No. 6,255,774; and U.S. Pat. No. 6,402,579; which are incorporated herein by reference in their entirety.
The organic luminescent materials used to make layers 36, 38, and 40, are sensitive to contamination, oxidation, and humidity. Furthermore, in some OLEDs, electrodes 34 and 42 are also sensitive to contamination, oxidation, and humidity. In the prior art, protective layer 44 actually comprises a glass or metal lid, which is attached to substrate 32 using a bead of UV-cured epoxy. Such an encapsulation technique is unsatisfactory. For example, an encapsulating lid requires additional bracing or standoffs for sizes greater then twelve centimeters on the diagonal.
U.S. Pat. No. 5,952,778 to Haskal et al. attempts to address the need in the art for encapsulating OLEDs in order to prolong device life. Haskal et al. disclose a three-layer protective covering. The first layer, nearest the cathode, is made of a stable metal such as gold, silver, aluminum or indium that is deposited by thin film deposition techniques. The second layer comprises a dielectric material such as silicon oxide or silicon nitride deposited by thermal or chemical vapor deposition. The third layer of the protective covering comprises a hydrophobic polymer such as siloxane or Teflon®. The three-layer protective covering described by Haskal et al. is unsatisfactory, however, because it is time consuming to manufacture the coverings described by Haskal et al.
Therefore, there is still a need in the art for techniques and methods for encapsulating organic electroluminescent devices such as OLEDs and PLEDs.